A conventional two-wheeled vehicle, such as a bicycle, has a relatively rigid frame which provides only minimal absorption of any forces that are transferred to the frame from irregularities in the surface over which the vehicle is being driven. This is acceptable in those situations where the vehicle is intended for use only on roads or tracks where the degree of irregularity of the surface is fairly limited. However, as the use of two-wheeled vehicles “off-road” has increased over recent years, the conventional, generally rigid rear suspensions have proved to be wholly unsatisfactory for reasons of both comfort and performance.
Suspension systems in general and for bicycles in particular, have two primary functions. They are provided to increase the comfort of the rider (and any passenger) and to increase the performance of the vehicle. Increasing the comfort for the rider can be defined as attenuating vibrations that are induced in the vehicle by irregularities in the ground surface over which the vehicle travels. The greater the amount of these ground induced vibrations that are transmitted through the frame of the vehicle to the rider, the less comfortable the rider will be. Also, the more vibration that is transmitted through to the rider, the greater the amount of effort that will be expended by the rider thereby resulting in fatigue setting in more quickly. Conversely, if the ground effect vibrations can be damped out by the frame of the vehicle, the rider will experience a more comfortable ride and will not tire so easily.
Secondly, suspension systems are provided to increase the vehicle's performance. Performance is closely related to control. The control of a vehicle during acceleration, cornering and braking is largely a function of maintaining a more or less constant contact of the vehicle's tires with the ground, regardless of the terrain over which the vehicle is travelling Thus, the second main purpose of suspension systems is to maintain contact of the vehicle with the ground, i.e., keeping the tires on the ground to provide better control of the vehicle to the rider. In bumpy terrain, unsuspended systems suffer from reduced control because the tires periodically lose contact with the ground. When an unsuspended tire hits a bump, it will tend to “hop” off of the ground momentarily, the extent of the time of noncontact being a function of both the height and configuration of the bump and the speed of the vehicle at the time of its contact with the bump. While in mid-air, the tire can do nothing to assist the rider in maintaining control of the vehicle. Thus, suspension systems serve to reduce ground hop and to maintain contact between the vehicle's tires and the ground.
In order to overcome these difficulties and to provide vehicles that are more adapted to use in environments where greater degrees of ground obstacles are encountered, designers have sought to provide shock absorbing rear wheel suspension systems for those vehicles. Such rear wheel suspensions systems on pedal powered bicycles can provide the rider with the benefits of a more comfortable ride as well as better control of the vehicle. The rear-wheel bicycle suspension system provides a more comfortable ride by absorbing the shocks incurred from encountering ground obstacles, rather than transmitting them through the frame to the rider. Also, by maintaining a greater degree of contact between the rear tire and the ground while travelling over such ground obstacles, the rear wheel suspension system can give the rider better control over the vehicle for accelerating, braking, and cornering.
For a rear-wheel suspension system to be suitable for use on a bicycle, it must be efficient. Ideally, a perfect rear-wheel suspension system would permit the rear wheel of the vehicle to move in reaction to ground forces that are incurred, but would not react similarly to the application of drive-train forces to the rear wheel. Unwanted movement within the suspension system resulting from drive train forces wastes the rider's energy. Bicycle suspension systems can be designed to react to ground forces and not to drive-train forces by careful placement of the real or virtual pivot around which the rear axle rotates.
Several variations of rear wheel suspension systems exist in the prior art. One of these suspension systems comprises attaching the rear wheel's axle directly to a member which pivots around a single main pivot point on the main frame. The pivotable member is then biased downwardly by a spring or shock absorber so as to act to attempt to maintain the rear wheel in contact with the ground. In such a system, the pivot point around which the rear axle rotates is simply the pivot point at which the swing-arm member is attached to the frame. This type of suspension benefits from being simple; however, physical limitations of the bicycle's structure restrict the possible locations of the main pivot on the frame. This limits the designer's ability to vary the rear wheel travel's path to obtain greater efficiency. If the fixed main pivot is placed correctly, this type of suspension system can be reasonably effective, at least for ground surface conditions that are not severe. However, because the possible locations for the main pivot are limited by the frame's geometry and other components such as the front derailleur, optimization of the placement of the main pivot can, for example, interfere with the functioning of the bicycle's front derailleur. As such, the main pivot in this type of suspension system is usually located such that the suspension system provides much less than optimal efficiency.
As has been explained previously, the primary functions of a vehicle's suspension system is to absorb the energy transferred into the vehicle when the vehicle encounters irregularities in the ground's surface and to damp out vibrations of the vehicle frame that are induced by those irregularities. However, suspension systems can also absorb energy from the vehicle's drivetrain. In other words, if a force is exerted through the drivetrain for the purpose of making the vehicle go forward, and that force instead results in a compression of the suspension system, that energy is lost. The ratio of the energy transferred to the vehicle from encountering the irregularities in the ground surface which is absorbed by the vehicle suspension system to the total energy absorbed by the suspension system is termed the “efficiency” of the suspension system. Thus, an efficient suspension system is one which absorbs as much energy from the ground as possible, while absorbing as little energy from the drivetrain as possible. Efficiency is particularly crucial in vehicles, such as bicycles, which have a limited amount of power available. On a bicycle, the more of the rider's energy that can be translated into forward motion rather than into suspension motion, the better. Hence, it is a goal of suspension systems, particularly for bicycles, to be as efficient as possible.
Thus, another type of prior art suspension system which has been proposed for alleviating some of the problems that were present in the single pivot suspension systems is one in which a so-called “four-bar” linkage is used to permit the location of the center of rotation of the axle of the rear wheel to be varied over the path through which that axle travels during the compression of the suspension system. In such a system, two different linkages or two pairs of linkages, are attached at two different locations to the main frame of the bicycle. A third member, to which is attached the rear axle, is pivotally engaged with each of these two linkages. In such a four-bar linkage suspension system, the center of rotation of the rear axle is not fixed, but rather it varies with the positions of the linkages as a function of the position of the rear wheel's axle. As such, in a four-bar linkage type of suspension system, the geometry of the path through which the axle of the rear wheel travels as the suspension system is compressed is defined by the location of the instantaneous center of rotation (“ICR”) of the four bar linkage. The ICR, around which the rear axle rotates, is located at the intersection of two lines, each of which extends through each of the two pivot points that are associated with each of the two linkages that are attached to both the main frame of the bicycle and the third member which supports the rear wheel. As the suspension system moves, the ICR changes, unlike the fixed, main pivot suspension systems.
Numerous examples of such prior art four-bar linkage suspension systems exist. One such system is shown in U.S. Pat. No. 6,244,610, which issued on Jun. 12, 2001, to Kramer-Massow. The Kramer-Massow '610 patent discloses a four-bar linkage suspension system in which it appears that an attempt was made to provide the benefits of the isolation of the rider from the shocks produced by the ground surface by using a four-bar linkage, while maintaining the movement of the ICR within a relatively small area such that the path of travel of the rear wheel'axle is substantially circular like that of the fixed pivot suspension systems. This results in a design where the ICR causes an excessive increase in chain length over the vertical travel of the rear wheel's axle. It is a phenomena of many suspension systems in bicycles that, as the suspension system is compressed, the distance between the axis of the chainrings in the bicycle's bottom bracket and the axis of the rear wheel varies, resulting in a change in the length of drive chain between the chainring and the rear derailleur. A excessive growth in chain length results in undesirable pedal feedback.
Pedal feedback is a force that is exerted on the chain when the suspension system compresses. On bicycles with suspension systems that involve an increase in chain length during system compression, the pedals will be caused to rotate backwards when the suspension compresses to make up for the increase in chain length that is required. The rider feels this “pedal feedback” as a tugging force in the drivetrain while attempting to pedal to move the bicycle forward. Pedal feedback, because it is not a pleasant feeling, tends to disturb the pedalling of the rider. As such, it disrupts the smooth flow of energy and power from the rider's feet through the pedals into the drivetrain and the rear wheel. On the other hand, some increase in chain length is beneficial in counteracting the suspension system's tendency to compress when the rider pushes down on the pedals.
Also, in the Kramer-Massow patent, the ICR is located well above the bicycle frame's bottom bracket, a point that is too high considering the short arc radius of the rear wheel's travel path. Such a pivot location results in a path of travel for the rear wheel's axle (“wheelpath”) that is initially non-vertical and has a backward component of movement. The backward component of movement along the wheelpath results in a high degree of chain length increase and thereby causes a deleterious amount of pedal feedback.
Similarly, another four-bar linkage suspension system in which the annunciated goal of the design is to provide a rear wheel travel that initially permits the rear wheel to react favorably to the shock of bumps in the terrain being traversed, and to thereafter follow a path that does not vary greatly from an arc centered about the vehicle's crank assembly such that the suspension system does not generate significant chain tension, is shown in U.S. Pat. No. 5,791,674, which issued on Aug. 11, 1998, to D'Aluisio. The D'Aluisio patent is believed to suffer from at least most of the same problems as the Kramer-Massow patent.
A second example of a type of prior art four-bar linkage suspension system is disclosed in U.S. Pat. No. 5,509,679, issued Apr. 23, 1996, U.S. Pat. No. 5,678,837, issued Oct. 21, 1997, and U.S. Pat. No. 5,899,480, issued May 4, 1999, all of which were issued to Leitner. The Leitner patents disclose a four-bar linkage suspension system which comprises a pair of lower links having a front pivot which is near to the bottom bracket of the main bicycle frame and a rear pivot which is located below the center of the rear wheel's axis.
In such a four-bar linkage suspension, the front pivot of the lower link member is usually located such that the chain passes above and below the lower link member, and the rear pivot of the lower link member is located below the axis of the rear wheel. This arrangement limits the possible positions of each of the front and rear pivots severely. As a result, the positioning of the ICR is very limited and cannot be used advantageously to control the wheelpath of the vehicle's rear wheel so to efficiently absorb the energy imparted to the vehicle by the terrain being traversed.
A further example of a such a four-bar linkage in the prior art in which the lower link member is located below the chain is found in U.S. Pat. No. 5,409,249, issued Apr. 25, 1995, and U.S. Pat. No. 5,441,292, issued Aug. 15, 1995, both of which issued to Busby. This design suffers from the same problems as do those disclosed in the Leitner patents.
U.S. Pat. No. 5,553,881, issued Sep. 10, 1996, U.S. Pat. No. 5,628,524, issued May 13, 1997, and U.S. Pat. No. 6,206,397, issued Mar. 27, 2001, to Klassen, disclose another prior art four-bar linkage type of suspension system in which an attempt is made to control the degree of chain length increase, which, according to Klassen, is necessary in order to compensate for pedal feedback. In the Klassen patents, the rear wheel is directed along a predetermined, generally S-shaped path as the suspension is compressed in order to control the degree of chain length change during the compression of the suspension system. This is accomplished by moving the ICR along a path which causes the both the chain length and the rate of change of the chain length to initially increase to a maximum in the middle of the wheelpath and thereafter to decrease over the remainder of the wheelpath. The Klassen patents define that this maximum or peak in the chain's length and rate of chain length growth is critical to the performance of the suspension system.
The Klassen patents teach a design in which a peak is produced in each of two curves that represent movement of the suspension system during its operation: (1) the plot of the chain length versus distance travelled along the rear wheel's path of travel; and (2) the plot of the rate of change of the chain length versus distance travelled along the rear wheel's path of travel. These peaks are achieved by locating the members of the four-bar linkage suspension system in a precise manner such that the movement of one of the linkage members dominates during one portion of the suspension's travel, whereas the opposing linkage member dominates during the remaining portion of the suspension system's movement. As a result of these limitations on the linkages, their precise locations relative to each other greatly restrict the possible location of the ICR's that are achievable with the linkages. As a result, the prior art Klassen designs have less than optimal ICR locations; that is, the ICR locations are too near to the rear of the bicycle. This causes them to provide none of the benefits to the rider that occur as a result of projecting the ICR's forward as is taught by the present invention.
Also, the S-shaped wheelpath of the suspension systems of the Klassen patents results in an inconsistent suspension system behavior which the present inventor believes to be undesirable. For example only, allowing the chain's length to become shorter at any during the travel of the suspension system is very clearly undesirable. A decreasing chainlength provides no benefit in negating the rider's downward thrust on the pedals. To the contrary, such a decrease can only exacerbate the problem. On the other hand, a suspension system with a continuously increasing chainlength is necessary in order for any beneficial negating effect to occur. Furthermore, the S-shaped wheelpath means that the performance of the suspension system is inconsistent throughout the travel of the suspension system. At some points in the suspension system's travel, the rate of chainlength is decreasing, while at other times, it is increasing. This inconsistency is quite undesirable from the rider's point of view. A suspension system which has a constant rate of change of chainlength is far more desirable, whether that rate is constant, increasing or decreasing.
Moreover, in the Klassen patents, the rear pivot of the lower link is located very close to the rear wheel's axis. The suspension system employs a dual eccentric pedal crank mechanism which is mounted adjacent to and just below the bicycle's bottom bracket. This mechanism creates the preferred S-shaped wheelpath of the rear wheel's axis. This acts to provide a chain lengthening effect in an attempt to counteract any suspension compression that is caused by the downward forces arising from the weight of the rider and from the pedalling forces. However, as the rear wheel's axis travels along the wheelpath, there is an increasing chain lengthening effect in the lower part of the wheelpath below an inflection point, and a decreasing chain lengthening effect through the upper part of the wheelpath. Additionally, the dual eccentric pedal crank mechanism adds considerable complexity to the bicycle, thereby significantly increasing the chance of reliability and maintenance problems, particularly in view of the terrain over which such bicycles are frequently ridden.
Another example of a prior art four-bar linkage suspension system is that which was shown in an advertisement for the Bianchi Super G mountain bike that appeared in Mountain Bike magazine, April 1996, at page 95. The Bianchi Super G bicycle incorporated a four-bar linkage in which, when in its at rest condition, the two opposing linkage members were each relatively short and generally parallel to each other with the lower linkage member being shorter than the upper member. Thus, this linkage caused the ICR to move radically, starting far out in front of the bicycle, initially moving forward to essentially infinity, and then switching rapidly to points which are far behind the bicycle. This radical movement of the ICR resulted in a wheelpath of the rear wheel's axle that was very curved due to the large changes in ICR location thereby causing a chain length increase that is initially very rapid, thereafter falling off rapidly, and even going negative at high degrees of compression of the suspension system. Such a highly curved wheelpath, with the large changes in ICR, and the rate of chain length increase and decrease resulted in a highly inefficient suspension system.
A still further example of a four-bar linkage in the prior art is found in U.S. Pat. No. 5,4352.910, issued Sep. 26, 1995, to Harris. The linkage shown and disclosed in the Harris patent is wholly different from all of those discussed so far in that the bottom bracket of the bicycle, i.e., the portion of bicycle which carries and supports the pedal cranks, is mounted on the swing arm of the four-bar linkage. As such, there is never any change in chain length. However, because the axis about which the pedals rotate is moving at all times during use of the bicycle, this design will be very disconcerting and tiring for the rider. Also, this design does not provide sufficient isolation of the rider from the shocks and vibration caused by irregularities in the terrain.
Finally, a recently issued patent to Ellsworth, U.S. Pat. No. 6,378,885, issued Apr. 30, 2002, discloses a bicycle suspension system utilizing a four-bar linkage which attempts to locate and maintain the instantaneous center of rotation of the bicycle's rear wheel as close as possible to a line that is defined by the tension side of the chain drive of the bicycle. The Ellsworth patent also attempts to maintain the instantaneous center of gravity well in front of the bicycle's bottom bracket in order to prevent “squatting” of the suspension system. In doing so, the Ellsworth patent utilizes upper and lower rocker arms that are of similar effective length and which are mounted at their rear ends to seat stay members that are nearly vertical in their attitude. Also, the lower rocker arm is located close to the rear wheel's axle at its rear end and adjacent to the bottom bracket at the front end. This arrangement enables the structure of the Ellsworth patent to have a locus of its instantaneous centers of rotation that does not move significantly, either vertically or horizontally, although, by necessity, the instantaneous centers of rotation do become somewhat lower as the suspension system is compressed. It is further noted that the Ellsworth patent attempts to have as little growth of chain length as is possible over the full range of compression. While it is clear that the Ellsworth patent is attempting to reach certain of the same goals as does the present invention (as is also the case with at least most of the aforementioned prior art), as will be seen from the following description, the principles and techniques used to accomplish these goals are quite different.
An additional disadvantage of many of the above suspension systems is the possibility of “chain suck” occurring. This occurs when the bicycle's chain gets caught between a gear chainring and a link of the suspension system and usually occurs when one of the link members is near or below the level of the driven length of the chain.